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The Gondwana margin: Proterozoic to Mesozoic

The longevity and extent of the oceanic southern of '--'H��'.JAJ.J.'u., E. I.M. Gonzalez-Casado and I.A. Dahlquist Gondwana have made it the of intense study for more on "The Maz terrane: a Mesoproterozoic domain in the western than 70 years. It was one of the cradles of terrane and Sierras equivalent to the remains a proving ground for theories of Antofalla block of southern Peru? for West

LL.L�""".LE, """'.LJL.LU''-'H and Investigation on this Gondwana evolution" sheds new light on the Middle margin, such as accretionary orogenesis and terrane analysis, is and Late Proterozoic evolution of the western Amazonia margin vital to our understanding of the Proterozoic and Phanerozoic that preceded final amalgamation of West Gondwana in the Late evolution of the continental crust. In this issue of Cambrian. The Maz terrane Gondwana Research, entitled "The West Gondwana Margin: Sierras Pampeanas) is recognised as a new continental terrane Proterozoic to Mesozoic", we have assembled 9 research papers that underwent Grenvillian-age orogeny and was thoroughly various of the evolution of the West the Ordovician Famatinian orogeny. Nd- and

Gondwana margin, frrst at the international .L.Ln-''-'�'J.�F. allows correlation of Maz metasedi­

'Gondwana 12 (Geological and Biological Heritage of Gond- rnp'nt�n"\T rocks with the Mesoproterozoic northern part of the

wana)', held in in November 2005. Many -'-'J.�V.L<-"HU craton, of pre-Andean basement in concern southern South which has a continuous southern Peru. These were probably continuous along the Proterozoic to Mesozoic geological record. palaeo-margin of the Amazonia craton, at least until the end of A Focus A.P.M. Vaughan and R.I. Pankhurst the Neoproterozoic. provides a "Tectonic overview of the West Gondwana The paper by A.B. Guereschi and R. D. Martino and It provides an defmition of West Gondwana based on textural evidence of two migmatization events in the Sierras de the cratonic the and Cordoba, Argentina") focuses on tectonothermal evolution in mobile Palaeozoic-Mesozoic terranes, and the the internal of the orogen where are boundary with East Gondwana. The history of formation and The authors introduce for the frrst time the pull­

'.UL>fJ".L.L>LU of the supercontinent is and each sector push concept for the Pampean orogen, i.e., a succession of of the oceanic receives a brief state-of-the-art review. cmnpresslOnal and extensional events, the tectonic Part of the discussion concentrates on the South American processes of subduction and collision of the Pampia terrane sector, where there are widely-debated hypotheses of collisional against the West Gondwana margin. In contrast to former views pre�-e)�lstmg continental crustal terranes (the of this being a short-lived Cambrian orogeny, and Chilenia terranes, and a starting some 30 to 50 Ma earlier, in the Late Voluminous silicic magmatic provinces characterise this Neoproterozoic.

the margin and abundant form the The paper by C.I. '-/u'-'.L.L.u".. J.L'-> E.O. Zappettini, I.O.S. Santos, outermost and of the South African E. and N.I. on "Foreland basin de-r:1oslts sector is treated next, followed by West Antarctica, with the associated with accretion in La Pampa most up-to-date summary so including the most recent presents a mUltidisciplinary study results from terrane studies of the Antarctic Peninsula. The fmal marine basin in western sections deal with New Victoria Land and the Curaco basin), whose are defmed by aeromagnetic

Transantarctic Mountains - of the East Gondwana data. Field and petrological studies show that the L>"'-".LLH''-'U�L.L.L that acted as sinks or sources of, West Gondwana sequence in the easternmost on Gondwana material or terranes. The paper fmishes with some continental crust, can be divided into two distinct unconform­ observations on possible future directions for studies of the able formations (Late Ordovician-Devonian and Permian,

Gondwana ��A __ ��.'�' ge()Cl1ermstry of the lower sequence The other contributions are ordered in terms of the indicates an active depositional environment, and U-Pb chronology of processes. The paper by C. Casquet, R.I. SHRIIvIP dating of detrital zircon confrrrns a Late Ordovician Pankhurst, C.W. Rapela, C. Galindo, C.M. M. depositional age, with provenance from a Cambrian (pampean) source. Hf isotope data on the zircons show that the source banded metalliferous and radiolarian metacherts, metapelites region was mature, but generally not as old as Palaeoproter­ and redeposited calcareous metasandstones of the Denaro ozoic. The authors interpret this as a foreland basin resulting complex, part of the Madre de Dios terrane. The basaltic rocks from Mid Ordovician collision of the Precordillera terrane plot in the N- and E-type MORB fieldsof tectonic discriminant (Cuyania), with sedimentation across the palaeo-suture. diagrams and were probably erupted along a constructive plate The paper by I. L. Alonso, I. Gallastegui, I. Garcia­ margin. They possess a foliation interpreted as having developed Sansegundo, P. Farias R. Rodriguez Femandez and V A. Rarnos during accretion of the terrane to the Gondwana margin. The on "Extensional tectonics and gravitational collapse in an authors argue that the structural and metamorphic data Ordovician passive margin: the western Argentine Precordil­ (pumpellyite-actinolite facies) suggest formation in an accre­ lera" describes ubiquitous extensional structures developed in tionary wedge at relatively low Tand P, probably during the Late Ordovician rocks in the Argentine Precordillera. These Triassic-Early Iurassic Chonide event. structures include normal faults and boudinaged sequences C. Adams ("Geochronology of Palaeozoic terranes at the that illustrate a range of deformational styles developed while Pacific Ocean margin of Zealandia") presents new geochrono­ the sediments were still soft during the early stages of logical data from the largely submerged part of East Gondwana lithification. Structural data support the interpretation that of which New Zealand is the largest emergent part. This gravitational collapse related to submarine sliding was the includes Lord Howe Rise, Challenger Plateau, the New Zealand cause of extensional deformation. The new data support earlier mainland itself, Chatham Rise and Campbell Plateau. The new conclusions locating an Ordovician continental slope between data suggest that that the Campbell Plateau is mostly underlain the ocean floorof the westemmost part of the Precordillera and by Early Palaeozoic metasediments intruded by (1) Early to the carbonate platform of the central Precordillera, interpreted mid-Cretaceous granitoids along the western margin, extending as a passive continental margin. north to New Zealand and south to Antarctica, and (2) Early The paper by B. Castro de Machuca, G. Arancibia, D. Iurassic granitoids of Bounty Platform, extending to Marie Morata, D. Belmar, L. Previley and S. Pontoriero on the "P-T-t Byrd Land. evolution of an Early Silurian medium-grade shear zone on the The final paper is by D.H. Elliot and C.M. Fanning: "Detrital west side of the Famatinian magmatic arc, Argentina: zircons from Upper Permian and Lower Triassic Victoria Group implication for the assembly of the western Gondwana margin" sandstones, Shackleton Glacier region, Antarctica: evidence for investigates the duration of the Famatinian tectono-thermal mUltiple sources along the Gondwana plate margin". This event that supposedly resulted from collision of the Laurentia­ presents new SHRIMP zircon data showing that the Victoria derived Precordillera terrane. The authors chose a shear zone Group sediments had a contemporaneous Late Permian within basic igneous rocks belonging to the Famatinian magmatic source, with subsidiary N eoproterozoic, Devonian 9 magmatic arc and determined an Early Silurian 40 ArP Ar age and Cambrian sources. Palaeocurrent flow directions indicate on amphibole, younger than Middle Ordovician peak-meta­ derivation of the Permian detritusfrom the West Antarctic flank morphic ages obtained by other workers. Their interpretation is of the Beacon foreland basin, the inferred Panthallassan plate that this age represents cooling through late orogenic uplift and margin. The Devonian source is attributed to the Ford decompression of the Famatinian mobile belt. Determination of granodiorite suite in Marie Byrd Land, West Antarctica. P-T conditions reinforce the view than Famatinian peak Because the Beacon basin strata cover the Cambrian Ross metamorphism was at high-T and intermediate-P orogen and extend onto older basement, the Cambrian and S.E. Geuna, L.D. Escosteguy and R. Miro in their paper Upper Proterozoic zircons are interpreted as reworked from entitled "Palaeomagnetism of the Late Devonian-Early Car­ Devonian and/or Lower Permian Beacon sandstones exposed as boniferous Achala Batholith, Cordoba, central Argentina: a result of the onset of folding and thrusting in Late Permian implications for the apparent polar wander path of Gondwana" time. argue for a mid-Palaeozoic age (380-360 Ma) for a palaeo­ In assembling this special issue of Gondwana Research, we magnetic pole derived from the Achala batholith, a major post­ acknowledge the major contribution made by the authors in orogenic complex emplaced in the metamorphic-plutonic submitting their manuscriptsand undertakingimporta nt revisions basement of the Eastern Sierras Pampeanas. The body, where necessary. We are all indebted to the following for consisting mainly of porphyritic to coarse-grained equigranular generously giving their time in order to provide scholarly and monzogranite, has a crystallization age of � 370 and a cooling painstaking reviews: I. Abati, I. Arribas, H. Bahlburg, E.G. age of � 340 Ma; it is exposed as asymmetric, eastward-tilted Baldo, L. Barbero,M.A. Basei, I.D. Bradshaw, I.M. Cebria, M.I. blocks. The palaeomagnetic pole based on 43 sites is located at Curtis, M. Flowerdew, I.D. Gleason, P. Gonzalez, D. Gregori, L.P. 56°S, 307°E and fits a complex mid-Palaeozoic apparent polar Gromet, P. Herve, I.P. Hibbard, I.L. Isbell, R.I. Korsch, P. T. Leat, wander path ("Y-type") for Gondwana, which may involve Iiabiao Li, E.I. Llambias, M. Lopez de Luchi, Xiwu Luan, H.-I. rapid movement, true polar wander episodes and/or continental Massonne, 1. Metcalfe, C. Mpodozis, M. Palin, S.H. Peralta, O. collisions before the final amalgamation of Pangea. Rabbia, A.E. Rapalini, I. Schwartz, L.A. Spalletti, R.A.I. Trouw, The paper by P.A. Sepulveda , P.Herve, M. Calderon and I. P. C.R. Van Staal, R. Varela, W. von Gosen, G. Vujovich, A. Lacassie on "Petrology of metamorphic and igneous units from Wandres, N. Woodcock, Nengyou Wu, U. Zimmermann. We the allochthonous Madre de Dios Terrane, Magallanes, Chile" thank Gondwana Research for support in publishing this special describes metamorphosed pillow basalts, m etahyal 0 clastite s, issue and hope that the papersassembled hereinwill contribute to state-of-the-art and continued research R.J. Pankhurst

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